function deshu(n){
    var sum = 0;
    var shu1 = document.querySelector("#num1").value;
    var shu2 = document.querySelector("#num2").value;
    shu1 = Number(shu1);
    shu2 = Number(shu2);
    switch (n){
        case 'jia':
            sum = shu1 + shu2;
        break;
        case 'jian':
            sum = shu1 - shu2;
        break;
        case 'cheng':
            sum = shu1 * shu2;
        break;
        case 'chu':
            sum = shu1 / shu2;
        break;
    }
      document.querySelector("#new").value = sum;
}